Metal organic frameworks for the capture of volatile organic compounds

ABSTRACT

The use of porous crystalline solids constituted of a metal-organic framework (MOF) for the capture of polar volatile organic compounds (VOCs). In particular, the MOF of interest are material having an average pores sizes of 0.4 to 0.6 nm and an hydrophobic core formed by a metal oxide and/or hydroxide network connected by linkers, the linkers being selected from the group including (i) C 6 -C 24  aromatic polycarboxylate linkers, such as benzyl or naphtyl di-, tri- or tetracarboxylate, and (ii) C 6 -C 16  polycarboxylate aliphatic linkers; the linkers bearing or not apolar fluorinated groups, e.g. —(CF 2 )n—CF 3  groups, n being a integer from 0 to 5, preferably 0 ou 3, and/or apolar C 1 -C 20  preferably C 1 -C 4  alkyl groups, e.g. —CH 3  or —CH 2 —CH 3 , grafted directly to the linkers and pointing within the pores of the MOF. The MOF solids used in the present invention can be used for the purification of air, for example for the capture of polar VOCs like acetic acid and aldehydes from indoor air in cars, museums and archives, much more efficiently than common adsorbents, particularly in presence of above normal levels of humidity. They can in particular be used for the preservation of cultural heritage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International PatentApplication Number PCT/EP2018/074571 filed on Sep. 12, 2018 which claimspriority to European Patent Application n° EP 17306170.6 filed on 12Sep. 2017, the entire contents of which said applications are herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates, inter alia, to the use of porouscrystalline solids constituted of a metal-organic framework (MOF) forthe capture of polar volatile organic compounds (VOCs).

The MOF solids of the present disclosure can be used for thepurification of air, for example for the capture of polar VOCs likeacetic acid and aldehydes from indoor air in cars, museums and archives,much more efficiently than common adsorbents, particularly in presenceof above normal levels of humidity. They can in particular be used forthe preservation of cultural heritage.

BACKGROUND

It is commonly admitted that polar VOCs like acetic acid oracetaldehydes are significant pollutants inside cars, museums andarchives. For example, in museums, polar VOCs need to be controlled, asreported for example in N. Blades et al., Guidelines on PollutionControl in Heritage Buildings, The Council For Museums, Archives AndLibraries, London, 2000. Cultural heritage conservation at museums andhistorical buildings depends indeed on the limitation of acetic acidpollution in indoor air and inside showcases, as reported for example inD. Thickett et al. in Met. 98 Proc. Int. Conf. Met. Conserv. (Eds.: W.Mourey, L. Robbiola), James & James, London, 1998, pp. 260-264, and V.Kontozova et al. in Proc. art 2002 7th Int. Conf. Nondestruct. Test.Microanal. Diagnostics Conserv. Cult. Environ. Herit. (Eds.: R. VanGrieken, K. Janssens, L. Van't clack, G. Meersman), University OfAntwerp, Antwerp, 2002 [2,3]. This volatile organic pollutant causesreduction in the degree of polymerization of cellulose in paper,corrosion of lead-containing alloys and other metals, and degradecalcareous materials (stones, ceramics), as disclosed for example in B.Krupinska, R. Van Grieken, K De Wael, Microchem. J. 2013, 110, 350. Itmay be readily formed from the degradation of woods, as reported in L.T. Gibson et al., Corros. Sci. 2010, 52, 172 and, thus, completeelimination of sources is often difficult, if not impossible, andregular monitoring the acetic acid concentration in air is beingproposed, as reported for example in T. Prosek et al. Corros. Sci. 2014,87, 376.

A very low maximum average concentration of acetic acid of 400 and 40ppb for a 1- and 100-year preservation target, respectively, for museum,gallery, library, and archival collections has been proposed in AmericanSociety of Heating Refrigeration and Air Conditioning Engineers, inHeating, Vent. Air-Conditioning Appl., ASHRAE, Atlanta, 2003.

One option for its removal is the use of adsorbent materials at museums.Classic adsorbents, like zeolites and activated carbons, have been muchapplied to capture volatile organic compounds. However, acetic acidposes additional challenges comparing with other less acidic compounds,due to the competitive adsorption of water under conventional humidityconditions of storage of cultural artefacts (which are above normalhumidity conditions, around 40% RH). Zeolites can have very polarsurfaces to strongly interact with acetic acid, but they are thenreadily saturated with water at very low humidity levels, as reported inM. L. Pinto, et al., Adsorption 2003, 9, 303. On the contrary, activatedcarbons are usually more hydrophobic than zeolites and are not saturatedwith water at relative humidity above 60%, but they do not strongly bindacetic acid.

Metal-organic frameworks are a versatile class of porous hybridcrystalline architectures, developed in the last decades, made from theassociation of inorganic moieties and polycomplexing organic linkers,forming micro or mesoporous materials whose pore size, shape, surfacearea and hydrophilic/hydrophobic balance can be tuned for a wide rangeof potential applications. In the field of adsorption, the benefits ofMOFs exhibiting active sites, like Lewis, Bronsted or redox orfunctional polar or apolar groups from the organic linkers, to interactspecifically with polar or quadrupolar molecules, e.g. CO, NO or CO₂, toenhance the selectivity towards more inert species (e.g. alkanes) can beunderlined. But nothing efficient has been reported yet about MOFs forefficiently capturing VOCs like acetic acid or acetaldehydes, especiallyin above normal humid environments. Further, there are still concernsabout the chemical and mechanical stability of this class of materials.

Therefore, there remains a need for finding stable means for capturingefficiently polar VOCs like acetic acid or acetaldehydes from indoor airin cars, museums and archives, especially in above normal humidenvironments, that outperform those of all the reported so far, and thatallow to preserve cultural heritages.

DETAILED DESCRIPTION

To address this need, extensive research have been conducted by presentinventors in order to find specific MOFs that are capable to veryefficiently adsorb polar VOCs like acetic acids or formaldehydes, evenin above normal humid environments.

Before addressing the description of the disclosure itself, in order tofacilitate an understanding of the present disclosure, a number of termsand phrases are defined here:

-   -   As used herein other than the claims, the terms “a,” “an,”        “the,” and/or “said” means one or more. As used herein in the        claim(s), when used in conjunction with the words “comprise,”        “comprises” and/or “comprising,” the words “a,” “an,” “the,”        and/or “said” may mean one or more than one. As used herein and        in the claims, the terms “having,” “has,” “is,” “have,”        “including,” “includes,” and/or “include” has the same meaning        as “comprising,” “comprises,” and “comprise.” As used herein and        in the claims “another” may mean at least a second or more. As        used herein and in the claims, “about” refers to any inherent        measurement error or a rounding of digits for a value (e.g., a        measured value, calculated value such as a ratio), and thus the        term “about” may be used with any value and/or range.    -   The phrase “a combination thereof” “a mixture thereof” and such        like following a listing, the use of “and/or” as part of a        listing, a listing in a table, the use of “etc” as part of a        listing, the phrase “such as,” and/or a listing within brackets        with “e.g.,” or i.e., refers to any combination (e.g., any        sub-set) of a set of listed components, and combinations and/or        mixtures of related species and/or embodiments described herein        though not directly placed in such a listing are also        contemplated. Such related and/or like genera(s), sub-genera(s),        specie(s), and/or embodiment(s) described herein are        contemplated both in the form of an individual component that        may be claimed, as well as a mixture and/or a combination that        may be described in the claims as “at least one selected from,”        “a mixture thereof” and/or “a combination thereof.”    -   In general, the term “substituted” whether preceded by the term        “optionally” or not, and substituents contained in formulae of        this disclosure, refer to the replacement of hydrogen radicals        in a given structure with the radical of a specified        substituent. When more than one position in any given structure        may be substituted with more than one substituent selected from        a specified group, the substituent may be either the same or        different at every position. As used herein, the term        “substituted” is contemplated to include all permissible        substituents of organic compounds.    -   As used herein, the term “about” refers to a variation of ±5 of        the value specified. For example, “about 50” percent can in some        embodiments carry a variation from 45 to 55 percent. As used        herein, the term “and/or” means any one of the items, any        combination of the items, or all of the items with which this        term is associated.    -   As will be understood by the skilled artisan, all numbers,        including those expressing quantities of ingredients, properties        such as cavity/pore size and BET specific surface area, reaction        conditions, and so forth, are approximations and are understood        as being optionally modified in all instances by the term        “about.” These values can vary depending upon the desired        properties sought to be obtained by those skilled in the art        utilizing the teachings of the descriptions herein. It is also        understood that such values inherently contain variability        necessarily resulting from the standard deviations found in        their respective testing measurements.    -   As will be understood by one skilled in the art, for any and all        purposes, particularly in terms of providing a written        description, all ranges recited herein also encompass any and        all possible subranges and combinations of subranges thereof, as        well as the individual values making up the range, particularly        integer values. A recited range includes each specific value,        integer, decimal, or identity within the range. Any listed range        can be easily recognized as sufficiently describing and enabling        the same range being broken down into at least equal halves,        thirds, quarters, fifths, or tenths. As a non-limiting example,        each range discussed herein can be readily broken down into a        lower third, middle third and upper third, etc.    -   One skilled in the art will also readily recognize that where        members are grouped together in a common manner, such as in a        Markush group, the disclosure encompasses not only the entire        group listed as a whole, but each member of the group        individually and all possible subgroups of the main group.        Additionally, for all purposes, the disclosure encompasses not        only the main group, but also the main group absent one or more        of the group members. The disclosure therefore envisages the        explicit exclusion of any one or more of members of a recited        group. Accordingly, provisos may apply to any of the disclosed        categories or embodiments whereby any one or more of the recited        elements, species, or embodiments, may be excluded from such        categories or embodiments, for example, as used in an explicit        negative limitation.    -   As used herein, the expression “three-dimensional structure” is        understood to mean a three-dimensional sequence or repetition of        units or subvariants, as is conventionally understood in the        field of MOF materials, that are also characterized as        “organometallic polymers”.    -   As used herein, the term “solid” refers to any type of        crystalline material. Said solid may be, for example, in the        form of crystals, powder or particles of varied forms, for        example of spherical, lamellar, etc. form. The particles may be        in the form of nanoparticles.

As used herein, “humid environment” means an atmosphere environmentcomprising water vapor. It can be the air with water vapor. The amountof water vapor present in the environment, e.g. air, increases as thetemperature increases. The differences in the amount of water vapor in aparcel of air can be quite large. For example, a parcel of air that isnear saturation may contain 28 grams of water per cubic meter of air at30° C., but only 8 grams of water per cubic meter of air at 8° C. Watervapor or vapour or aqueous vapor is the gaseous phase of water. It isone state of water within the hydrosphere. Under typical atmosphericconditions, water vapor is continuously generated by evaporation andremoved by condensation. The vapor content of air may be measured withdevices known as hygrometers. In the present disclosure, the amount ofwater vapor in the environment, e.g. air, may be from above dry air tosaturation, for example from 10 to 30° C., for example at roomtemperature, i.e. at a temperature between 18° C. and 28° C. As usedherein, the term “above normal humid environment” refers to a humiditylevel of an environment, as defined above, that is above theconventionally accepted threshold for normal humid environment for humancomfort. This “normal humidity” threshold is typically around 25-30%humidity. Cf. Ashrae Fundamentals Handbook, SI Edition, 2001, p. 24.5.[34] In the context of the present disclosure, an above normal humidenvironment refers to >30% relative humidity, preferably ≥35% relativehumidity, still preferably ≥40% relative humidity, up to 100% relativehumidity. Indoor environments such as museums, galleries, libraries, andarchival collections are conventionally associated with above normalrelative humidity levels around 40% RH. The additional constraint forbest preservation of the items stored in these places is a low maximumaverage concentration of acetic acid, typically between 400 and 40 ppb.

-   -   MOFs are constructed from bridging organic ligands, also named        “linkers” or “linker” or “spacers” or “spacer” that remain        intact throughout the synthesis, these ligands acting as linkers        in the network of the obtained MOF structure. As used herein,        the term “ligand” or “linker” or “spacer” refers to a ligand        coordinated to at least two metals, which participates in        providing distance between these metals and in forming empty        spaces or pores, named also “core” in the MOF.

The present inventors have shown through the present disclosure, that itis possible to strongly enhance the capture efficiency and confinementof the VOCs, especially polar VOCs like acetic acids or formaldehydes,even in an above normal humid environment, by using a particularselection of hydrophobic MOFs, having a controlled pore size and builtup from particular polycarboxylic alkyl or aromatic linkers, e.g. benzylor naphtyl dicarboxylate, and/or including grafted hydrophobicperfluoro, e.g. CF3, or alkyl, e.g. C1-C3, groups pointing within thepores of the MOF and grafted directly on the linkers or aromaticspacers.

In particular, the present disclosure relates to the use of a porousMetal-Organic Framework (MOF) material, for the adsorption of polarvolatile organic compounds, comprising an average pore size of 0.4 to0.6 nm and an hydrophobic core formed by a metal oxide and/or hydroxidenetwork connected by linkers, said linkers being selected from the groupcomprising:

-   -   C₆-C₂₄ aromatic polycarboxylate linkers, such as benzyl or        naphtyl di-, tri- or tetracarboxylate, and    -   C₆-C₁₆ polycarboxylate aliphatic linkers;

the linkers bearing or not apolar fluorinated groups, e.g.—(CF₂)_(n)—CF₃ groups, n being an integer from 0 to 5, preferably 0 ou1, and/or apolar C₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or—CH₂—CH₃, grafted directly to the linkers and pointing within the poresof the MOF.

The MOF linker may be a C₄-C₁₆ polycarboxylate alkyl linkers (such asdi-, tri- or tetracarboxylate or carboxylic acid linkers, for exampleC₂H₂(CO₂ ⁻)₂ (fumarate), C₂H₄(CO₂ ⁻)₂ (succinate), C₃H₆(CO₂ ⁻)₂(glutarate), (C₄H₄)(CO₂ ⁻)₂ (muconate), C₄H₈(CO₂ ⁻)₂ (adipate)),optionally bearing apolar fluorinated groups, e.g. —(CF₂)_(n)—CF₃groups, n being a integer from 0 to 5, preferably 0 ou 1, and/or apolarC₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or —CH₂—CH₃, grafteddirectly to the linkers and pointing within the pores of the MOF.

The MOF linker may be an imidazole-based linker (such as in ZIF-8 MOFmaterial), optionally bearing apolar fluorinated groups, e.g.—(CF₂)_(n)—CF₃ groups, n being a integer from 0 to 5, preferably 0 ou 1,and/or apolar C₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or—CH₂—CH₃, grafted directly to the linkers and pointing within the poresof the MOF.

Advantageously, the present disclosure provides the use of a porousMetal-Organic Framework (MOF) material, as defined above, for theadsorption of polar volatile organic compounds present at aconcentration in the range of 10 ppb to 100 ppm in a gaseousenvironment, such as air, with >30% relative humidity, preferably ≥35%relative humidity, still preferably ≥40% relative humidity. The porousMetal-Organic Framework (MOF) material, as defined above, may even beused for the adsorption of polar volatile organic compounds present at aconcentration as low as 10 ppb to 10 ppm in a gaseous environment, suchas air, at above normal relative humidity levels (e.g. >30% RH,preferably ≥35% RH, still preferably ≥40% RH).

As used herein, the term “average pore size” will be understood to referto the MOF pore size (or pore diameter), as conventionally used in theart, as calculated by the nitrogen adsorption method. It is meant toencompass the various possible pore geometries of the MOF material‘e.g., tetrahedral, octahedral). Methods for measuring the pore size arewell documented in the literature. Cf. Rouquérol, F.; Rouquérol, J.;Sing, K. Adsorption by powders and porous solids; Academic Press: SanDiego, 1999. [39] For example, the MOF pore size may be determined bycalculation of the pore size distributions from nitrogen adsorption.Alternatively, maximum and limiting pore sizes may be estimated fromcrystallographic data by simulating the filling of the pores with gasmolecules (L. Sarkisov and A. Harrison, Mol. Simul., 2011, 37, 1248-1257[40]) that allow the calculation of average pore sizes.

The present disclosure relates also to a process for adsorbing polarvolatile organic compounds present in an environment comprising the stepof contacting with said environment a porous Metal-Organic Framework(MOF) material, for the adsorption of polar volatile organic compounds,comprising an average pores sizes of 0.4 to 0.6 nm and an hydrophobiccore formed by a metal oxide and/or hydroxide network connected bylinkers, said linkers being selected from the group comprising:

-   -   C₆-C₂₄ aromatic polycarboxylate linkers, such as benzyl or        naphtyl di-, tri- or tetracarboxylate, and    -   C₆-C₁₆ polycarboxylate aliphatic linkers;

the linkers bear or not apolar fluorinated groups, e.g. —(CF₂)_(n)—CF₃groups, n being a integer from 0 to 5, preferably 0 ou 1, and/or apolarC₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or —CH₂—CH₃, grafteddirectly to the linkers and pointing within the pores of the MOF.

As mentioned above, the MOF linker may be a C₄-C₁₆ polycarboxylate alkyllinkers (such as di-, tri- or tetracarboxylate or carboxylic acidlinkers, for example C₂H₂(CO₂ ⁻)₂ (fumarate), C₂H₄(CO₂ ⁻)₂ (succinate),C₃H₆(CO₂ ⁻)₂ (glutarate), (C₄H₄)(CO₂ ⁻)₂ (muconate), C₄H₈(CO₂ ⁻)₂(adipate)), optionally bearing apolar fluorinated groups, e.g.—(CF₂)_(n)—CF₃ groups, n being a integer from 0 to 5, preferably 0 ou 1,and/or apolar C₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or—CH₂—CH₃, grafted directly to the linkers and pointing within the poresof the MOF.

In another example, the linker may be an imidazole-based linker (such asin ZIF-8 MOF material), optionally bearing apolar fluorinated groups,e.g. —(CF₂)_(n)—CF₃ groups, n being a integer from 0 to 5, preferably 0ou 1, and/or apolar C₁-C₂₀ preferably C₁-C₄ alkyl groups, e.g. —CH₃ or—CH₂—CH₃, grafted directly to the linkers and pointing within the poresof the MOF.

Advantageously, the apolar fluorinated groups may all be covalentlybonded to the ligands from the MOF structures. Alternatively, the apolarfluorinated groups may be grafted directly on the metal clusters. In yetanother alternative, the MOF pores could be impregnated with moleculesbearing apolar fluorinated groups, that could stay on the pores, beingpoorly volatile at room temperature. Teachings of these differentmethods may be found for example in the following references:

Coordination to the Open Metal Sites:

Hwang, Y. K.; Hong, D.-Y.; Chang, J.-S.; Jhung, S. H.; Seo, Y.-K.; Kim,J.; Vimont, A.; Daturi, M.; Serre, C.; Férey, G. Amine grafting oncoordinatively unsaturated metal centers of MOFs: consequences forcatalysis and metal encapsulation. Angew. Chem. Int. Ed. 2008, 47 (22),4144-4148. [35]

Exchange of Cation and Ligand After the Synthesis of MOFs:

Kim, M.; Cahill, J. F.; Fei, H.; Prather, K. A.; Cohen, S. M.Postsynthetic Ligand and Cation Exchange in Robust Metal-OrganicFrameworks. J. Am. Chem. Soc. 2012, 134 (43), 18082-18088. [36]

MOFs Postsynthetic Modifications:

a) Zhou, H.-C. “Joe”; Kitagawa, S. Metal-Organic Frameworks (MOFs).Chem. Soc. Rev. 2014, 43 (16), 5415-5418; [37] and b) Tanabe, K. K.;Cohen, S. M. Postsynthetic modification of metal-organic frameworks—aprogress report. Chem. Soc. Rev. 2011, 40 (2), 498-519. [38]

Advantageously, the polar volatile organic compounds may be selected inthe group comprising acetic acid, acetaldehyde, formaldehyde or amixture of two or three thereof. These polar volatile organic compoundsmay be adsorbed according to the present disclosure even in above normalhumid or moisted environments, even far above the 40% relative humidityfound in museums, as shown in the examples below.

Advantageously, the C₆-C₂₄ aromatic polycarboxylate linkers, such asbenzyl or naphtyl di-, tri- or tetracarboxylate may be selected from thegroup comprising C₆H₄(CO₂ ⁻)₂ (terephtalate), C₁₀H₆(CO₂ ⁻)₂(naphtalene-2,6-dicarboxylate), C₁₂H₈(CO₂ ⁻)₂(biphenyl-4,4′-dicarboxylate), C₆H₃(CO₂ ⁻)₃(benzene-1,2,4-tricarboxylate), C₆H₃(CO₂ ⁻)₃(benzene-1,3,5-tricarboxylate), C₂₄H₁₅(CO₂ ⁻)₃(benzene-1,3,5-tribenzoate), C₆H₂(CO₂ ⁻)₄(benzene-1,2,4,5-tetracarboxylate, C₁₀H₄(CO₂ ⁻)₄(naphtalene-2,3,6,7-tetracarboxylate), C₁₀H₄(CO₂ ⁻)₄(naphtalene-1,4,5,8-tetracarboxylate), C₁₂H₆(CO₂ ⁻)₄(biphenyl-3,5,3′,5′-tetracarboxylate), and modified analogues selectedfrom 2-methyl terephthalate, 2,5-dimethyl terephtalate, tetramethylterepthalate, perfluoromethyl terephtalate, diperfluoromethylterephthalate, 2-chloroterephthalate, 2-bromoterephthalate, 2,5tetrafluoroterephthalate, tetrafluoroterephthalate,dimethyl-4,4′-biphenyldicarboxylate,tetramethyl-4,4′-biphenyldicarboxylate,dicarboxy-4,4′-biphenyldicarboxylate, azobenzene dicarboxylate, orazobenzene tetracarboxylate.

Advantageously, the C₄-C₁₆ polycarboxylate alkyl linkers, may be di-,tri- or tetracarboxylate or carboxylic acid linkers, for exampleC₂H₂(CO₂ ⁻)₂ (fumarate), C₂H₄(CO₂ ⁻)₂ (succinate), C₃H₆(CO₂ ⁻)₂(glutarate), (C₄H₄)(CO₂ ⁻)₂ (muconate), or C₄H₈(CO₂ ⁻)₂ (adipate).

Advantageously, the above-defined linkers may bear or not apolarfluorinated groups, e.g. —(CF₂)n—CF₃ groups, n being a integer from 0 to5, preferably 0 or 1, for example —CF₃. The number of the selectedapolar fluorinated groups may be from 1 to 3 per linker.

Advantageously, the above-defined linkers may bear or not apolar C₁-C₂₀,preferably C₁-C₄, alkyl groups (named herein, e.g. —CH₃ or —CH₂—CH₃,grafted directly to the linkers and pointing within the pores of theMOF. The number of the selected apolar alkyl groups may be from 1 to 3per linker.

Advantageously, the MOF my bear apolar fluorinated groups as definedabove and apolar alkyl groups as defined above.

Advantageously, the metal atom of the metal oxide and/or hydroxide maybe selected from Li, Na, Rb, Mg, Ca, Sr, Ba, Sc, Ti, Zr, Ta, Cr, Mo, W,Mn, Fe, Ru, Os, Co, Ni, Pd, Pt, Cu, Au, Zn, Al, Ga, In, Si, Ge, Sn, Bi,Cd, Mn, Tb, Gd, Ce, La, or Cr. Preferably, the MOF material may be azirconium, zinc, iron, aluminum, chromium or their hydroxide based MOF.

Advantageously, the MOF may for example be selected from the groupcomprising MIL-140B, MIL-140C, UiO-66-2CF₃, UiO-NDC, UiO-66-(CH₃)₂, ZIF,for example ZIF-8, MIL-53, MIL-53-(CF₃)₂, MIL-69 and MIL-88B-4CH₃;preferably from the group comprising MIL-140B, MIL-140C, UiO-66-2CF₃,UiO-NDC, UiO-66-(CH₃)₂, MIL-53, MIL-53-(CF₃)₂, or MIL-88B-4CH₃. UiO-NDC(also referred to as “DUT-52”) refers to a UiO-type MOF with1,4-naphthalenedicarboxylate linkers.

MIL-140B and MIL-140C from (Institut Lavoisier) are porous zirconiumdicarboxylic MOFs pertaining to the MIL-140(B/C/D) series constructedfrom Zr oxide chains consisting of ZrO₇ polyhedra that are connected insix directions through aromatic dicarboxylate linkers to definetriangularly shaped microporous one-dimensional channels. The generalformula of the MOF MIL-140B is [ZrO(O₂C—C₁₀H₆—CO₂)₆]. Document V.Guillerm et al. Angew. Chemie Int. Ed. 2012, 51, 9267-9271 disclosesthese MOFs and examples for protocols for manufacturing the same.

UiO-66-2CF₃ and UiO-NDC are both obtained from the MOF UiO-66(Universitetet i Oslo) of formula [Zr₆O₄(OH)₄(O₂C—C₆H₄—CO₂)₆] made up of[Zr₆O₄(OH)₄] clusters with 1,4-benzodicarboxylic acid struts. DocumentJ. H. Cavka et al., J. Am. Chem. Soc., 2008, 130, 13850-13851 [10]discloses examples of protocols to obtain UiO MOFs. UiO-66-2CF₃ may forexample be obtained by the protocol disclosed in Q. Yang et al. Chem.Commun. 2011, 47, 9603-9605 [11] and S. Biswas et al. Eur. J. Inorg.Chem. 2013, 12, 2154-2160 [12] UiO-NDC (NDC for1,4-naphthalenedicarboxilic acid), also known in the literature asDUT-52, may for example be obtained by the protocol disclosed indocuments V. Guillerm et al. Angew. Chemie Int. Ed. 2012, 51, 9267-9271and V. Bon et al. CrystEngComm 2013, 15, 9572-9577 [13].

UiO-66-(CH₃)₂ is also a metal organic framework made up of [Zr₆O₄(OH)₄]clusters with 1,4-benzodicarboxylic acid struts. Document Yuting Huanget al., Enhanced stability and CO2 affinity of a UiO-66 typemetal-organic framework decorated with dimethyl groups, Dalton Trans.,2012, 41, 9283-9285 [14] discloses an exemple of protocol that can beused to obtain UiO-66-(CH₃)₂.

ZIF (Zeolitic Imidazolate Framework) is a metal organic framework (MOF)made by zinc ions coordinated by four imidazolate rings in the same wayas Si and Al atoms are covalently joined by bridging oxygens inzeolites. ZIF, for example ZIF-8, may be prepared by solvothermal orhydrothermal techniques. Document K. Park et al. Proc. Natl. Acad. Sci.2006, 103, 10186-10191 [15] discloses this MOF and protocols formanufacturing the same.

MIL-53 is a metal organic framework of formula M(C₈H₃O₄)(OH) made ofmetal III ions (M=Cr, Al, Fe, Ga, In) coordinated by1,4-benzodicarboxylic acid, which make three-dimensional porous solidsbuilt up from chains of corner-sharing MO₄(OH)₂ octahedra, connectedthrough 1,4-benzodicarboxylate linkers to define diamond-shapedone-dimensional channels. Documents T. Loiseau, et al. Chem. Eur. J.2004, 10, 1373-1382 [16] or C. Serre et al. J. Am. Chem. Soc. 2002, 124,13519-13526 [17] or T. R. Whitfield et al. J. Solid State Sci. 2005, 7,1096-1103 [18] or E. V. Anokhina et al. J. Am. Chem. Soc. 2005, 127,15000-15001 [19] disclose example protocols of manufacturing the same.MIL-53 materials can be grafted with functional groups, preferably withperfluoro and alkyl groups on the 1,4-benzodicarboxylate linkers, withinthe spirit of the present disclosure to strongly enhance the captureefficiency of polar VOCs. Examples of such materials areMIL-53(Fe)—(CF₃) and MIL-53(Fe)—(CH₃) that may be obtained by exampleprotocols disclosed in document T. Devic et al. J. Am. Chem. Soc. 2010,132 (3), 1127-1136 [20].

MIL-69 is a metal organic framework of formula Al(OH)(O₂C—C₁₀H₆—CO₂).H₂Omade by by using 2,6-naphthalenedicarboxylic acid as a rigid ligand. Thecrystal structure consists of infinite chains of AlO₄(OH)₂ octahedracorner-linked through the μ2-hydroxyl groups and connected by the2,6-naphthalenedicarboxylate moieties. It results in the formation offlat channels parallel to the chains of aluminum octahedra, runningalong the c axis. Document T. Loiseau et al., Hydrothermal Synthesis andCrystal Structure of a New Three-Dimensional Aluminum-Organic FrameworkMIL-69 with 2,6-Naphthalenedicarboxylate (ndc), AI(OH)(ndc)QH₂O, C. R.Chimie, Special Issue on Crystalline and Organized Porous Solids, 8,765-72 (2005) [21] discloses example protocols of manufacturing thesame.

MIL-88B(Fe)-4CH₃ is an hexagonal flexible iron(III) dicarboxylatematerial built up from iron(III) oxocentered trimers of iron octahedraand tetramethyl terephtalate linkers resulting into 1D microporouschannels, of pore size close to 6 Angströms, decorated with methylgroups from the linker and terminal water molecules from the inorganicsubunit. Its BET surface area is close to 1500 m²/g while its flexiblecharacter is limited to 25% in unit cell volume. The reader may refer toRamsahye, Naseem; Khuong Trung, Thuy; Scott, Lorna; Nouar, Farid; Devic,Thomas; Horcajada, Patricia; Magnier, Emmanuel; David, Olivier; Serre,Christian; Trens, Philippe “Impact of the flexible character of MIL-88iron(III) dicarboxylates on the adsorption of n-alkanes” Chem. Mater.,2013, 25, (3), 479-488 for a disclosure of protocols of manufacturingthe same. [41]

Advantageously, the MOF may preferably be under a form allowing a largeexchange surface between the MOF and the environment where the polarvolatile organic compounds have to be captured by adsorption. The MOFmay for example be in the form of a powder or granules or embedded inthe form of a composite material, embedded in or applied onto thesurface of a paper sheet or a polymer or a fiber. For example, documentWO2009/123484 [22] published on October 2009 discloses a useful processfor producing polyurethane foam filter material with adsorptioncapabilities that can be used to support the MOF to carry out thepresent disclosure. Other examples are the electropining of polymercontaining MOF particles disclosed in documents M. Rose et al. Adv. Eng.Mater. 2011, 13, 356-360 [23], R. Ostermann at al. Chem. Commun. 2011,47, 442-444 [24], J. Ren et al. Int. J. Hydrogen Energy 2015, 40,9382-9387 [25] and M. R. Khan et al. J. Mater. Eng. Perform. 2016, 25,1276-1283 [26] that give final composite fiber materials with supportedMOF that simplify the application of MOF for the adsorption of volatileorganic compounds.

A stark advantage of the present disclosure is that the MOFs arespecially designed for removing noxious VOC (volatile organic compounds)from air at very low concentrations in the presence of a highconcentration of water vapor.

As porous materials, MOFs have been used as adsorbent materials andfilters. MOFs can adsorb some significant amount of water and organiccompounds such as acetic acid or acetaldehyde as expected from a porousmaterial. For instance, MOFs have been tested to remove saturatedamounts of VOC and odors from air to demonstrate the MOFs' highretention capacity. However, no solution exists for the efficientremoval of polar VOC at low concentration in above normal humidenvironments. This is because the competing adsorption of water (a polarmolecule) hampers the use of MOFs for adsorbing polar volatile organiccompounds that are present in very low concentration. The presence ofwater strongly influences the adsorption of polar VOC when theconcentration of VOC to remove is very low. No solution exists so far.The present disclosure is thus the very first report of a solution tosignificantly improve conditions to remove polar VOC present in very lowconcentrations (both in absolute and relative pressures) when watervapor is present in high concentrations. The present disclosuretherefore concerns the use of MOFs that are specially designedchemically and structurally for enhanced/improved adsorption of polarvolatile organic compounds present at low concentration in above normalhumid environments. The MOFs described in the present document thereforepresent a good stability to moisture, and renders possible the improvedadsorption of polar volatile organic compounds at concentration in therange of 10 ppb to 100 ppm in an environment >30% relative humidity,preferably ≥35% relative humidity, still preferably ≥40% relativehumidity, up to 100% relative humidity; or even for the adsorption ofpolar volatile organic compounds present at a concentration as low as 10ppb to 10 ppm in a gaseous environment, such as air, at above normalrelative humidity levels.

The present disclosure allows advantageously the easy removal of lowconcentrations of acetic acid from indoor air, even is the presence ofabove normal water levels in the atmosphere/environment, for example inmuseums where it poses serious conservation problems. The followingexperiments and results confirm the benefits of the present disclosure,over existing solutions.

The following representative examples and figures are intended to helpillustrate the disclosure, and are not intended to, nor should they beconstructed to, limit the scope of the disclosure. Indeed, variousmodifications of the disclosure and many further embodiments thereof, inaddition to those shown and described herein, will become apparent tothose skilled in the art from the full contents of this document,including the examples which follow and the references to the scientificand patent literature cited herein. It should further be appreciatedthat the contents of those cited references are incorporated herein byreference to help illustrate the state of the art.

The following examples contain important additional information,examplification and guidance that can be adapted to the practice of thisdisclosure in its various embodiments and the equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows the obtained nitrogen adsorption isotherms, at −196° C.,on a) UiO-66, UiO-66-NH₂, UiO-66-2CF₃ and UiO-NDC and MIL-53(Fe)-2CF₃;and b) MIL-101-Cr, MIL-101-Cr—NH₂, MIL-140A, MIL-140B and ZIF-8.

FIG. 2: shows, acetic acid adsorption isotherms, at 25° C., expressed asequivalent liquid adsorbed volume.

FIG. 3: shows the nitrogen and water adsorption isotherms results, at−196° C. and 30° C. respectively, on: a) UiO-66, UiO-66-NH₂, UiO-66-2CF₃and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH₂, MIL-140A and MIL-140B.Adsorbed volume assuming a liquid like state at the adsorptiontemperature.

FIG. 4: shows water adsorption isotherms results, at 30° C., on: a)UiO-66, UiO-66-NH₂, UiO-66-2CF₃ and UiO-NDC; and b) MIL-101-Cr,MIL-101-Cr—NH₂, MIL-140A and MIL-140B, amounts expressed by surface areaof each material

FIG. 5: shows: a) results of a blanc experiments with no injection ofacetic acid (baseline) and the injection of acetic acid without thepresence of adsorbents. b) results with acetic acid concentrationprofiles inside a closed chamber after the injection of 1 μL of aceticacid, at 23° C., 40% RH, with the presence of 50 mg of MIL-140A,MIL-140B, MIL-140C, UiO-66-2CF₃, MIL-53-2CF₃ and ZIF-8.

FIG. 6: shows acetaldehyde concentration profiles results inside aclosed chamber after the injection of 14 of acetaldehyde, at 23° C., 40%RH, with the presence of 50 mg of UiO-66-2CF₃, MIL-140B, and a standardcommercial activated carbon RB4. The best results are obtained forMIL-140B and C UiO-66-2CF₃.

FIG. 7: shows nitrogen adsorption isotherms, at −196° C., of UiO-66-2CH₃of Example 6.

FIG. 8: shows acetic acid concentration profiles results inside a closedchamber after the injection of 1 μL of acetic acid, at 27° C., 40% RH,with the presence of 50 mg of UiO-66-2CH₃.

EXAMPLES

According to the present disclosure, the usable MOFs materials and theirpreparation can be understood further by the examples that illustratesome of the processes by which these materials are prepared or used. Itwill be appreciated, however, that these examples should not beconstrued to limit the disclosure. Variations of the disclosure, nowknown or further developed, are considered to fall within the scope ofthe present disclosure as described herein and as hereinafter claimed.

Example 1 Materials Synthesis

1.1. MIL-101(Cr): MIL-101(Cr) was obtained via a hydrothermal treatmentof a mixture of terephthalic acid (166 mg, 1 mmol), Cr(NO₃)₃.9H2O (400mg, 1 mmol), HF (0.2 mL, 1 mmol) and deionized water (4.8 mL, 265 mmol)heated up to 220° C. for 8 h as disclosed in document G. Férey, et al.Science 2005, 309, 2040-2042 [27]. After cooling down the autoclave, agreen powder can be removed and washed. Removing the large excess ofunreacted terephthalic acid from the powder is performed by followingthis purification process. First, with a glass filter whose the poresize is between 40 and 100 μm, the solution is filtered off twice toremove insoluble terephthalic acid from the solution. Then, the productis put into an autoclave to be washed with ethanol at 80° C. for 24 h.After this step, the solid is mixed into a solution of 1M of NH₄F at 70°C. for 24 h followed by a filtration and a wash with hot water. Theresulting product is then dried overnight at 150° C.

1.2. MIL-101(Cr)-EN: In order to get ethylenediamine graftedMIL-101(Cr), after dehydratation of 0.5 g of MIL-101(Cr) heated at 150°C. for 12 h, the solid was suspended in 30 mL of anhydrous toluene, asdisclosed in document Y. K. Hwang, et al. Angew. Chem. Int. Ed. Engl.2008, 47, 4144-4148 [28]. Afterwards, ethylenediamine (0.05 mL, 0.75mmol) was added to the suspension and stirred under reflux for 12 h.After the reaction, the material was filtered out, washed with deionizedwater and ethanol, and dried at room temperature (i.e. in the presentexamples at a temperature between 18° C. and 28° C.).

1.3. ZIF-8: For the synthesis of ZIF-8, Zn(NO₃)₂.6H₂O (2.933 g, 9.87mmol) was firstly solubilized in 200 mL of methanol. The same operationwas carried out for the ligand by putting 2-methylimidazole (6.489 g,79.04 mmol) into 200 mL of methanol, as disclosed in document A.Demessence et al., Adsorption properties in high optical qualitynanoZIF-8 thin films with tunable thickness, J. Mater. Chem., 2010, 20,7676-7681 [29]. After the solubilization of the species, the solutionwith the metal was quickly poured into the ligand mixture under stirringat room temperature (i.e. in the present examples at a temperaturebetween 18° C. and 28° C.). Slowly, the solution became lesstranslucent. After 1 h, the reaction was stopped and the solid wasseparated from the liquid by centrifugation for 15 min at 20000 rpm. Theparticules were then washed with absolute ethanol and centrifugatedthree times to remove the excess of unreacted salt and ligand. The solidwas then dried at room temperature (i.e. in the present examples at atemperature between 18° C. and 28° C.) overnight.

1.4. UiO-66: UiO-66(Zr) was synthesized by mixing ZrCl₄ (5.825 g, 25mmol), terephthalic acid (8.300 g, 50 mmol), HCl (1.54 mL, 50 mmol, 37%)in 150 mL of N,N-Dimethylformamide (DMF). The solution was thentransferred into a 750 mL Teflon liner and heated overnight at 220° C.in oven. The solid obtained was filtered off, washed with DMF twicefollowed by two washed with acetone and dried at room temperature (i.e.in the present examples at a temperature between 18° C. and 28° C.).

1.5. UiO-66-NH₂: In order to get UiO-66-NH₂, a solution composed ofZrCl₄ (233 mg, 1 mmol) and 2-aminoterephthalic acid (181 mg, 1 mmol) wasprepared in 3 mL of DMF and put into a 23 mL Teflon liner, as disclosedin document C. Gomes Silva, et al. Chem. Eur. J. 2010, 16, 11133-11138[30]. The mixture was heated in oven at 100° C. for 24 h. The solid wasthen recovered after filtration and treated with DMF. The material wasthen left in DMF at room temperature (i.e. in the present examples at atemperature between 18° C. and 28° C.) overnight under stirring. After anew filtration, the solid was washed twice with THF and dried at roomtemperature (i.e. in the present examples at a temperature between 18°C. and 28° C.).

1.6. UiO-66-2CF₃: The synthesis conditions of UiO-66-2CF₃ was like tothose of UiO-66 previously mentioned. ZrCl₄ (582 mg, 2.5 mmol),diperfluoromethyl terephthalic acid (595 mg, 2.5 mmol), HCl (0.077 mL,2.5 mmol, 37%) were dissolved and mixed in a 125 mL Teflon liner andthen heated in oven at 100° C. for 24 h. The obtained product wasfiltered off followed by two washes with DMF and two others with THF.The product was left at room temperature (i.e. in the present examplesat a temperature between 18° C. and 28° C.) overnight for theevaporation of the solvent.

1.7. UiO-NDC: In a 50 mL round-bottom flask, UiO-NDC was obtained byfirstly solubilizing 2,6-naphthalenedicarboxylic acid (1.296 g, 6 mmol)in 108 mL of DMF at 90° C. as disclosed in document S. Kaskel et al.,Cryst Eng Comm 2013, 15 (45), 9572-9577 [31]. Afterwards, benzoic acid(7.32 g, 60 mmol) and HCl (0.98 mL, 32 mmol, 37%) were put into thesolution followed by ZrCl₄ (1.398 g, 6 mmol). The mixture was left at90° C. for 6 h. The solid was separated by centrifugation and washedthree times with DMF and twice with EtOH.

1.8. MIL-140A: ZrCl₄ (13.980 g, 60 mmol), terephthalic acid (19.96 g,120 mmol) and acetic acid (34 mL, 600 mmol) were put into a 500 mLround-bottom flask filled with 400 mL of DMF as disclosed in document V.Guillerm et al. Angew. Chemie Int. Ed. 2012, 51, 9267-9271. The solutionwas kept under reflux and stirring for 6 h. The solid obtained afterfiltration was washed first with 1 L of DMF at 120° C. for 2 h30,filtrated and washed a last time with 1.5 L of MeOH for 12 h.

1.9. MIL-140B: In a 500 mL round-bottom flask, ZrCl₄ (2.77 g, 11.9mmol), 2,6-naphthalenedicarboxylic acid (7.08 g, 32.8 mmol) and aceticacid (25.5 mL, 450 mmol) were mixed in 430 mL of DMF and kept underreflux for 7 h. The solution was then filtrated and the resulting solidwas washed with 200 mL of DMF at 120° C. for 2 h30 followed withfiltration and a last wash with 400 mL of MeOH for 12 h to get afterfiltration the material of interest, MIL-140B.

1.10 MIL-53-2CF₃: In a 100 mL Teflon-lined reactor, 0.755 g (2.5 mmol)of 2,5-diperfluoroterephthalic acid, 0.675 g (2.5 mmol) of iron(III)chloride hexahydrate and 25 mL of deionized water were mixed. Theresulting mixture is, stirred for ten minutes followed by placing thereactor in a microwave heated to 100° C. for a 20 min period (heatingrate 60° C./min). The product is a yellow crystalline solid that can berecovered by centrifugation, and dried in air. The activation was doneby heating at 250° C. under vacuum for two days.

Example 2 Materials Characterization

Three series of analysis have been carried out to confirm the obtentionof the various materials synthetized in above Example 1:

-   -   nitrogen adsorption at −196° C.;    -   PXRD patterns of the synthesized materials; and    -   Thermogravimetric analysis of the synthesized materials.

2.1 Nitrogen adsorption at −196° C.: Nitrogen (Air Liquid, 99.999%)adsorption-desorption isotherms were measured at −196° C. using a liquidnitrogen cryogenic bath, in a volumetric automatic apparatus(Micromeritics, ASAP 2010). Prior to the measurement the samples wereoutgassed at 150° C. for 8 h at a pressure lower than 0.133 Pa.

2.2 PXRD patterns of the synthesized materials: The X-ray powderdiffraction patterns were obtained with a high resolution D5000 SiemensX'Pert MDP diffractometer (λCu, Kα1, Kα2) from 5 to 20° (2θ) using astep of 0.02° and 10 s of accumulation per step in continuous mode.

2.3 Thermogravimetric analysis of the synthesized materials: In order toget the TGA profile of each material synthesized, the sample (about 10mg) were analysed with a gravimetric analyser (Model Perkin Elmer STA6000) in air at a constant rate of 2° C./min.

All results of these analysis confirm the obtention of the variousmaterials synthetized in example 1.

Example 3 Experiments on Adsorption Capacity of the Different Materials

3.1. Protocol Used for Water Adsorption Measurements

The adsorption isotherms of water were determined at 30.0±0.1° C. in anautomated apparatus, model Omnisorp 100cx (Coulter, USA), using a fixeddosing method. All samples were outgassed at 150° C. during 4 h at avacuum lower than 10⁻² Pa. The amounts adsorbed on an empty cell wereused to correct the data of the adsorption isotherms.

3.2. Protocol Used for Acetic Acid Adsorption Measurements

Adsorption isotherms of were measured by the volumetric method at lowrelative pressure, up to 0.06 p/p⁰, on about 50 mg different samples ofthe materials synthetized in example 1, each outgassed as describedabove. Adsorption temperature was maintained with a water bath (GrantGD120) at 25° C. The pressure was measured with a capacitance transducerfrom Pfeiffer Vacuum (CMR 262). Nonideality of the phase was accountedby the use of the compressibility factor z, expressed as function of thepressure p, given by the equation z=0.351+0.729 p^(−0.176), which wasobtained by fitting data published in document F. H. MacDougall, J. Am.Chem. Soc. 1936, 58, 2585 [32].

3.3. Protocol Used for Acetic Acid Adsorption in a Controlled RelativeHumidity Environment

About 100 mg of materials were placed inside 10 cm³ glass vials and keptin an oven at 100° C. over night. The vials were removed and closedtightly with polyethylene caps until being used in the experiments. 50mg of adsorbent material was weighted (Mettler AE240) in a watch glassand immediately placed inside a glass chamber (2.9 dm³) with controlledhumidity. The humidity was controlled to about 40% relative humidity bymeans of a saturated solution of potassium carbonate (BDH Prolabo,99.6%) in an open petri dish, as disclosed in document L. Greenspan, J.Res. Natl. Bur. Stand. Sect. A Phys. Chem. 1977, 81A, 89 [33]. Thechamber was flushed with nitrogen flow during 15 minutes and thematerial was allowed to equilibrate with the humidified atmosphere for1.5 hour. After this time, a syringe (Hamilton 7001 KH) was used toinject 1 μL of acetic acid (Riedel-de Haën, 99.8%) inside the chambertrough a rubber septum injection port, in the surface of a clean paperfilter to improve the spreading and evaporation of the small droplet.Immediately before injection of acetic acid, the humidifier withpotassium carbonate was removed to assure that acetic acid removal isonly due to the tested MOF. The total volatile organic compounds (TVOC)concentration, temperature and relative humidity inside the chamber weremeasured (Graywolf TG-502 TVOC ppb) and were recorded at fixed timeinterval (15 s) during one hour using computer software (Wolfsense LAP).During experiments, the temperature was 22.8±0.8° C. and the relativehumidity was 39.6±3.5%. A blank experiment with no injection of aceticacid was preformed and a control experiment was preformed with injectionof acetic acid without any MOF to demonstrate the tightness of thechamber during the experiments time frame, and ascertain the TVOC signalresponse obtained by 1 μL injection.

Example 4 Results of the Analysis

4.1 Nitrogen Adsorption

Annexed FIG. 1 shows the obtained nitrogen adsorption isotherms, at−196° C., on a) UiO-66, UiO-66-NH₂, UiO-66-2CF₃ and UiO-NDC; and b)MIL-101-Cr, MIL-101-Cr—NH₂ MIL-140A, MIL-140B and ZIF-8.

4.2 Acetic Acid Adsorption

Annexed FIG. 2 shows, acetic acid adsorption isotherms, at 25° C.,expressed as equivalent liquid adsorbed volume.

4.3 Water Adsorption and Assessment of Hydrophobic/Hydrophilic Characterof Surfaces

The water adsorption isotherms represented on annexed FIG. 3 display thedifferent hydrophobic/hydrophilic character of the different materials.This figure shows nitrogen and water adsorption isotherms, at −196° C.and 30° C. respectively, on: a) UiO-66, UiO-66-2CF₃ and UiO-NDC; and b)MIL-101-Cr, MIL-101-Cr—NH₂, MIL-140A and MIL-140B, adsorbed volumeassuming a liquid like state at the adsorption temperature. The wateramounts adsorbed in each material were not dependent on the pore volume(or surface area), in agreement with the results reported by otherauthors for some of the studied MOF and demonstrating the stronginfluence of the chemical nature of the surface on the results. Also,the different shapes of the isotherms indicate this strong influence.For example, for UiO-66 and UiO-66-NH₂ that have similar microporousvolume and pore sizes, the absorbed amounts at 0.7 p/p⁰ are very similar(about 22 mmol g⁻¹), but the inflection points of both isotherms occurat significantly different relative pressures (FIG. 3). The UiO-66-NH₂exhibit one jump at 0.25 p/p⁰, while UiO-66 exhibit two consecutiveinflection points at 0.35 and 0.45 p/p⁰. On the contrary, UiO-66-2CF₃displays a lower adsorbed amount than UiO-66 and an inflexion point atabout 0.5 p/p⁰. Such comparison of inflection points in water isothermsproved helpful for evaluating porous materials' hydrophobicity withdifferent types of surface chemistry and can be correlated with theinteraction energy of water with the surface. Thus, results indicatethat functionalization of UiO-66 with amines decreases thehydrophobicity of the material, while the contrary occurs with thepresence of CF₃ groups. The changing of benzene to naphthalenedicarboxylate linker increases considerably the hydrophobicity of thistype of structure as can be seen from the comparison of the UiO-66 andUiO-NDC isotherms (FIG. 3).

Further confirmation of this change in water affinity can be seen if theresults are represented as amounts adsorbed by surface area of materialsinstead of mass, as shown on annexed FIG. 4. FIG. 4 shows wateradsorption isotherms, at 30° C., on: a) UiO-66, UiO-66-NH₂, UiO-66-2CF₃and UiO-NDC; and b) MIL-101-Cr, MIL-101-Cr—NH₂, MIL-140A and MIL-140B,amounts expressed by surface area of each material. This representationis better for comparing the nature of the surface of materials withsignificantly different specific surface areas. The results depicted inannexed FIG. 4 show that at all the studied materials of the UiO-66family tend to about the same covering of the surface (between 0.020 and0.025 mmol m⁻²) at high relative pressures, but they are significantlydifferent below the 0.6 p/p⁰ pressure region, being UiO-2CF₃ the mosthydrophobic one (FIG. 4).

In MIL-101-Cr water adsorption isotherm (FIG. 3), an initial step below0.1 p/p⁰ attributed to the adsorption of water in the open metal sitesand Cr metal clusters is observed, followed by an intermediate plateau.The second inflection is observed at a high relative pressure (between0.6 to 0.75 p/p⁰) and the magnitude of the step indicates that most ofthe surface of the material is hydrophobic. Comparing the amountsadsorbed by surface area on the MIL-101-Cr with those of the UiO-66family (FIG. 4), it can be concluded that the former is adsorbing muchless water per surface. In fact, the nitrogen and water isotherms can becompared considering the adsorbed phase in a liquid like state and usethe respective liquid density to compare the adsorbed volumes (FIG. 3).The volumes of water adsorbed on the materials of the UiO-66 typeapproach those obtained with nitrogen (FIG. 3), although it occurs atdifferent relative pressures for each material. Nevertheless, we canconclude that the micropore volume of these materials become filled withwater at pressures above 0.6 p/p⁰. On the contrary, for MIL-101-Cr it isevident that, even at 0.8 p/p⁰ (after the second inflection point), theporous volume is still far from being completely filled with water (FIG.3).

The main differences observed in water adsorption between theMIL-101-Cr—NH₂ and the parent MIL-101-Cr are the disappearance of thefirst inflection point at low pressures and a slow raise in the adsorbedamounts without a defined step at intermediate pressures, for the aminefunctionalized material. The absence of the first inflection point ismost surely related with the presence of the ethylenediamine thatoccupies the open metal sites, were adsorption occurs at very lowpressures. But, the amine groups decrease the hydrophobicity of thematerial since the adsorbed amounts per surface are higher than thoseobserved for the parent MIL-101-Cr, at intermediate pressures (FIG. 4).Nevertheless, at high pressures the pore volume the MIL-101-Cr—NH₂ (FIG.3) is still not saturated, similarly to the behaviour of MIL-101-Cr.FIG. 4 puts in evidence the more hydrophobic nature of MIL-101-Cr andMIL-101-Cr—NH₂ materials in relation to the UiO-66 type materials.

The MIL-140A and MIL-140B adsorb significantly less water amounts thanthe other tested materials. Even when taking into account the lowsurface area of this material, the amounts adsorbed per surface aresignificantly below the other materials, except ZIF-8 (FIG. 4),confirming the hydrophobic nature of these materials, which was studiedby infrared techniques. Although they are formed by the same type ofmetal clusters and linker as UiO-66 and UiO-NDC, the structural featuresof MIL-140A and MIL-140B renders a much more hydrophobic surface. ZIF-8presents very low adsorbed amounts confirming the hydrophobic characterof this MOF, being the most hydrophobic MOF from the tested samples.This can also be confirmed by the analysis of FIGS. 3 and 4.

4.4 Removal of Acetic Acid with MOFs in the Presence of Moisture

Annexed FIG. 5 shows: a) Blanc experiments with no injection of aceticacid (baseline) and the injection of acetic acid without the presence ofadsorbents. b) Acetic acid concentration profiles inside a closedchamber after the injection of 1 μL of acetic acid, at 23° C., 40% RH,with the presence of 50 mg of MIL-140A, MIL-140B MIL-140C, UiO-66-2CF₃and ZIF-8. Regarding the MIL-140 family of materials, FIG. 5 shows anincrease in the efficiency of removal from the MIL-140A to the MIL-140B,because the concentration inside the chamber after one hour issignificantly lower in the latter case. When going from the MIL-140B tothe larger pores MIL-140C the efficiency of the removal decreases. Theseresults show that MIL-140B have the most suitable properties among theMIL-140 family, by a combination of proper pore size and hydrophobicity,for the removal of acetic acid. FIG. 5b demonstrates the advantage ofthe presence of the perfluoro methyl groups in the structure of theMOFs. The UiO-66-2CF₃ is the most efficient of the cases presented inFIG. 5b , even more than the very hydrophobic ZIF-8. The effect of thesegroups in MIL-53-2CF₃ is only noticed after some time because thisstructure need to change from the closed pore (initial) to the open pore(final) form. This justifies the sharp rise followed by a sharp decreasein the concentration.

Comparing the Adsorption Isotherm Data with the Data Obtained in theClosed Chamber

The Henry's constant reflects the affinity of the MOFs for acetic acidand can be used to estimate adsorbed amounts at very low concentrations.These amounts can be compared with the values estimated in the closedchamber experiments. The values calculated from the concentrations afterone hour (assuming equilibrium and no influence from the presence ofwater) for the best materials (Table 1 below) are ranging from 4.57 to0.18 μmol g⁻¹. Comparing with the amounts deduced from the closedchamber measurements (Table 1), one estimates that ZIF-8 is approachingequilibrium.

Remarkably, the estimations based on the acetic acid isotherm areagreeing fairly with observations preformed under the presence ofmoisture, probably due to the very hydrophobic character of ZIF-8 whichprevents the interference of moisture with the acetic acid adsorption.On the contrary, for UiO-66-2CF₃ and MIL140B, differences between theamount estimated from the isotherms and the one measured in the chamber(Table 1) are significant and indicate a strong influence of water onthe acetic acid adsorption and that the systems are not close toequilibrium after one hour. In fact, a considerable slope of theconcentration profiles inside the chamber at 1 hour is seen, which leadsto a drop to 4.2 μg dm⁻³ and 9.5 μg dm⁻³ after 1.5 hour, for UiO-66-2CF₃and MIL140B respectively.

TABLE 1 Comparison of the concentration of acetic acid in the chamberafter one hour, respective relative pressure and the adsorbed amounts inthe materials. Concentration n^(ads) n^(ads) in the chamber Relativefrom Henry's from FIG. after 1 hour pressure constants ^(a)) 1 ^(b))Material μg dm⁻³ p/p⁰ μmol g⁻¹ μmol g⁻¹ UiO66- 7.2 1.41 × 10⁻⁴ 4.57 0.202CF₃ UiO-NDC 20.5 4.00 × 10⁻⁴ 0.83 0.18 MIL-140B 12.2 2.38 × 10⁻⁴ 4.470.19 ZIF-8 20.2 3.94 × 10⁻⁴ 0.18 0.18 ^(a)) Adsorbed amounts in thematerials estimated from the Henry's constant, at the relative pressureafter one hour, assuming equilibrium and only acetic acid adsorption;^(b)) Adsorbed amounts in the materials estimated from the differencebetween the concentration in the chamber with materials and the blankexperiment, after one hour.

4.5 Removal of Acetaldehyde with MOFs in the Presence of Moisture

Annexed FIG. 6 shows acetaldehyde concentration profiles inside a closedchamber after the injection of 14 of acetaldehyde, at 23° C., 40% RH,with the presence of 50 mg of UiO-66-2CF₃, MIL-140B, and activatedcarbon RB4. The best results are obtained for MIL-140B and UiO-66-2CF₃.

Example 5 Results

The removal of low concentrations of acetic acid from indoor air atmuseums poses serious conservation problems that current adsorbentscannot easily solve due to the competitive adsorption of water. In thiswork, several topical MOFs with different pore sizes, topologies andpending functional groups have been studied to demonstrate what featuresare more effective to the challenge of capturing this very polarvolatile organic compound in the presence of water. Results show thatalthough increasing the hydrophobicity can have a positive effect in theremoval efficiency, it is not sufficient if not accompanied by anincreased interaction with acetic acid. The two best materials, MIL-140Band UiO-66-2CF₃, confirm that two strategies are possible to increaseselectively the interaction with acetic acid. For MIL-140B, thehydrophobicity combined with the proper pore width promote the aceticacid adsorption by a confinement effect. In the UiO-66-2CF₃, the aceticacid adsorption was enhanced by the introduction of the CF₃ groups thatincrease the hydrophobicity and the interaction with acetic acid.

Example 6 UiO-66-2CH₃

Synthesis Procedures

The reaction mixture of 178.13 mg (1 mmol) of Zirconyl chlorideoctahydrate, (98%), 194.18 mg (1 mmol) of 2,5-Dimethylterephthalic acid(97%) and 3.77 mL (100 mmol) of Formic acid (99%) were dispersed in 8.05mL (104 mmol) of dimethylformamide (98%). The mixture was placed in aTeflon-lined autoclave (23 mL) for 24 hours at 150° C. Then, the whitesolid was recovered by centrifugation and washed 3 times with 50 mL ofethanol.

Experimental Section

The X-ray powder diffraction patterns were obtained with a highresolution D5000 Siemens X'Pert MDP diffractometer (λ_(Cu), Kα₁, Kα₂).Thermogravimetric analysis was performed with a thermogravimetricanalyzer (Model Perkin Elmer STA 6000) in air at a constant heating rateof 2° C./min. Transmission IR spectra were measured using Nicolet 6700spectrometer. Nitrogen physisorption isotherms were measured at T=77Kwith a Micromeritics 3Flex surface characterisation analyser. Prior tothe measurements, the powders (50-80 mg) were outgassed for 6 h atT=373K under a 10⁻¹ mbar vacuum.

Results

The successful synthesis of UiO-66-2CH₃ was confirmed by powder X-raydiffraction (PXRD) studies. Characteristic peaks at 2θ=7.34°, 8.48°confirm UiO-66 structure. The UiO-66-2CH₃ has a face-centered cubic(fcc) unit cell, space group: Fm-3m.

The IR spectrum of UiO-66-2CH₃ showed:

-   -   an absorption band at 1574 cm⁻¹ indicating the existence of the        reaction of COOH with Zr⁴⁺,    -   the aromatic bound C═C from ligand is referred at 1493 cm⁻¹ to        C═C from aromatic; and    -   bands at 2969 and 2933 cm⁻¹ representing asymmetric stretchings        of methyl groups.

Using thermogravimetric analysis (TGA) the thermal stability ofUiO-66-2CH₃ was investigated. Three weight loss steps were observed. Thefirst weight loss of 3.4 wt % occurred between 20 and 60° C. due tovaporization of water and ethanol. The second step of weight loss was4.1 wt % at 60-300° C. due to dehydroxylation of OH⁻. The third step ofweight loss was 49 wt % at 300-550° C. due to decomposition of material.

FIG. 7 shows N₂ adsorption isotherms collected on the material. BETsurface area is 1563±8 m²/g, the maximum pore volume is 0.625566 cm³/gand the median pore width: 5,231 Å. It can be seen that UiO-66-2CH₃presented higher surface area (calculated using the BET theory), thantheoretical value 1200 m²/g. The higher surface area can be explained bypresence of the defects in the structure, like «missing linker defects»(which incidentally also explains its surface area higher than the bareUiO-66 MOF). The presence of this type of defects is attributed to ahigh degree of connectivity of the clusters.

In addition, UiO-66-2CH₃ (bearing methyl groups, as compared to UiO-66)enxhibits an enhanced/improved capture performance of acetic acid. FIG.8 represents the decrease of the acetic acid concentration as a functionof time, when 1 μL is injected in the chamber (as in the protocol ofexample 3, at 27° C.). The UiO-66-2CH₃, which is the equivalentstructure of UiO-66 but with two methyl groups per linker, thereforepresents an advantage over the structure without methyl groups. Inaddition, the UiO-66-2CH₃ material is advantageous due to the easypreparation and consequently lower production costs (the carboxylic acidused for the UiO-66-2CH₃ synthesis (terephthalic acid with two methylsubstitution on the aromatic ring; (HCO₂)₂—C₆H₂—(CH₃)₂) is availablecommercially) than the equivalent UiO-66-2CF₃ with perfluoro groupsinstead of methyl groups. The results for UiO-66-2CH₃ were obtained at atemperature (27° C.) slightly higher than those for UiO-66 (23° C.),implying that a slightly better comparative performance is expected forUiO-66-2CH₃ if the acetic acid capture is performed at the sametemperature.

It is proposed that alternative synthesis routes to UiO-66-2CH₃ (withoutinhibitors such as monocarboxylic acids) may lead to a lower defectcontent, a lower surface area but a more hydrophobic character, that mayimprove significantly the acetic acid capture of this MOF.

The invention claimed is:
 1. A process for adsorbing polar volatileorganic compounds present at a concentration in the range of 10 ppb to100 ppm (volume/volume) in a gaseous environment comprising the step ofcontacting with said environment a porous Metal-Organic Framework (MOF)material comprising an average pore size of 0.4 to 0.6 nm and anhydrophobic core formed by a metal oxide and/or hydroxide networkconnected by linkers, said linkers being selected from the groupconsisting of: C₆-C₂₄ aromatic polycarboxylate linkers; C₆-C₁₆polycarboxylate aliphatic linkers; C₄-C₁₆ polycarboxylate aliphaticlinkers; and imidazole-based linkers; each of the aforementioned linkersoptionally bearing apolar fluorinated groups and/or apolar C₁-C₂₀ groupsgrafted directly to the linkers and pointing within the pores of theMOF.
 2. The process according to claim 1, wherein the C₆-C₂₄ aromaticpolycarboxylate linkers are selected from the group consisting ofC₆H₄(CO₂ ⁻)₂ (terephthalate), C₁₀H₆(CO₂ ⁻)₂(naphthalene-2,6-dicarboxylate), C₁₂H₈(CO₂ ⁻)₂(biphenyl-4,4′-dicarboxylate), C₆H₃(CO₂ ⁻)₃(benzene-1,2,4-tricarboxylate), C₆H₃(CO₂ ⁻)₃(benzene-1,3,5-tricarboxylate), C₂₄H₁₅(CO₂ ⁻)₃(benzene-1,3,5-tribenzoate), C₆H₂(CO₂ ⁻)₄(benzene-1,2,4,5-tetracarboxylate, C₁₀H₄(CO₂ ⁻)₄(naphtalene-2,3,6,7-tetracarboxylate), C₁₀H₄(CO₂ ⁻)₄(naphtalene-1,4,5,8-tetracarboxylate), C₁₂H₆(CO₂ ⁻)₄(biphenyl-3,5,3′,5′-tetracarboxylate), and modified analogues selectedfrom 2-methyl terephthalate, 2,5-dimethyl terephthalate, tetramethylterephthalate, perfluoromethyl terephthalate, diperfluoromethylterephthalate, 2-chloroterephthalate, 2-bromoterephthalate,2,5-tetrafluoroterephthalate, tetrafluoroterephthalate,dimethyl-4,4′-biphenyldicarboxylate,tetramethyl-4,4′-biphenyldicarboxylate,dicarboxy-4,4′-biphenyldicarboxylate, azobenzene dicarboxylate, andazobenzene tetracarboxylate.
 3. The process according to claim 1,wherein the C₄-C₁₆ polycarboxylate alkyl linkers are selected from di-,tri- and tetracarboxylate or carboxylic acid linkers.
 4. The processaccording to claim 2, wherein the linkers optionally bear apolarfluorinated —(CF₂)—CF₃ or —CF₃ groups grafted directly to the linkersand pointing within the pores of the MOF.
 5. The process according toclaim 1, wherein the linkers optionally bear —CH₃ or —CH₂—CH₃, groupsgrafted directly to the linkers and pointing within the pores of theMOF.
 6. The process according to claim 1, wherein the metal atom of themetal oxide and/or hydroxide is selected from Li, Na, Rb, Mg, Ca, Sr,Ba, Sc, Ti, Zr, Ta, Cr, Mo, W, Mn, Fe, Ru, Os, Co, Ni, Pd, Pt, Cu, Au,Zn, Al, Ga, In, Si, Ge, Sn, Bi, Cd, Mn, Tb, Gd, Ce, La, and Cr.
 7. Theprocess according to claim 1, wherein the MOF is selected from the groupconsisting of MIL-140B, MIL-140C, UiO-66-2CF₃, UiO-NDC, UiO-66-(CH₃)₂,ZIF, ZIF-8, MIL-53, MIL-69 and MIL-88B-4CH₃.
 8. The process according toclaim 1, wherein the polar volatile organics compounds are selected fromthe group consisting of acetic acide, acetaldehyde, formaldehyde and amixture of two or three thereof.
 9. The process according to claim 1,wherein the MOF is in the form of a powder or granules or embedded inthe form of a composite material, or embedded in or applied onto thesurface of a paper sheet or a polymer or a fiber.
 10. The processaccording to claim 1, wherein the gaseous environment is air having >30%relative humidity.
 11. The process according to claim 1, wherein thelinkers are selected from the group consisting of: C₆-C₂₄ aromaticpolycarboxylate linkers, C₆-C₁₆ polycarboxylate aliphatic linkers, andC₄-C₁₆ polycarboxylate aliphatic linkers.
 12. The process according toclaim 1, wherein the C₆-C₂₄ aromatic polycarboxylate linkers are benzylor naphthyl di-, tri- or tetracarboxylates.
 13. The process according toclaim 1, wherein the apolar fluorinated groups are —(CF₂)_(n)—CF₃groups, n being an integer from 0 to
 5. 14. The process according toclaim 1, wherein the apolar C₁-C₂₀ groups are C₁-C₄ alkyl groups. 15.The process according to claim 13, wherein the apolar C₁-C₂₀ groups are—CH₃ or —CH₂—CH₃.
 16. The process according to claim 3, wherein theC₄-C₁₆ polycarboxylate alkyl linkers are selected from C₂H₂(CO₂ ⁻)₂(fumarate), C₂H₄(CO₂ ⁻)₂ (succinate), C₃H₆(CO₂ ⁻)₂ (glutarate),(C₄H₄)(CO₂ ⁻)₂ (muconate), and C₄H₈(CO₂ ⁻)₂ (adipate).
 17. The processaccording to claim 7, wherein the MOF is selected from the groupconsisting of MIL-140B, MIL-140C, UiO-66-2CF₃, UiO-NDC, UiO-66-(CH₃)₂,MIL-53, and MIL-88B-4CH₃.